Green’s Dictionary of Slang

swan n.

(US) a swan-dive.

1987

R. Campbell Alice in La-La Land (1999) 18: ‘Anybody ever do a swan?’ she asked.

In compounds

swan-hopping (n.) [in SE, a var. of swan-upping thus ? early play on up v. (3)]

adulterous sexual intercourse.

1683

Fifteen Real Comforts of Matrimony 95: You ha’ been to visit the Taylors Wife, I see by your hang-dog countenance […] I’le spoil your swan-hopping.